Switching power supplies have been increasingly utilized because of their high efficiency, their smaller size and their lower cost. To achieve power regulation, some of these switching supplies had a regulator and/or preregulator circuit which controlled the amount of energy applied to the inverter. Others achieved regulation by applying a fixed amount of energy to the inverter and varying the pulse width of the signal to the controlling devices of the inverter.
These prior supplies had the disadvantage that large amounts of rfi were generated by these switching elements. Also, these power systems were complex in both types of circuits and the number of circuit elements required.
In accordance with the preferred embodiment of the present invention, a fixed amount of energy is input to the inverter circuit. The frequency of the control signal to the inverter circuit is varied to achieve regulation. Because of the continuous operation of the inverter and the fact that small changes in the frequency of the driving signal to the inverter are all that were required to achieve regulation rather than the abrupt and discontinuous transitions used to control previous supplies, a much lower level of rfi is generated. This much lower level of electromagnetic interference (EMI) is a direct result of the absence of the high di/dt which occurs when an inverter is driven in the pulse width modulated mode. The lower level of di/dt also means there is less stress on the circuit components which are used. For example, in the preferred embodiment, the frequency of the driving signal is only varied between 18 KHz to 27 KHz to achieve power regulation. The resulting circuit is much less complex in both the number and types of circuits used, and hence is much more economical than previous supplies. Also, there is no requirement for other active components to achieve regulation since the output ripple is extremely low due to the almost pure sine wave output from the inverter .